The invention relates to the transfer of packet-switched data to a wireless terminal.
PDP contexts (Packet Data Protocol) are used in the transfer of user data in GPRS services (General Packet Radio Service) and in packet-switched services of the UMTS system (Universal Mobile Telecommunications System). PDP contexts are generally logical connections, on which IP data are transferred from a mobile station to a boundary node (GGSN) in a UMTS network and vice versa. For the mobile station, a PDP address (at least one) is specified, for which several PDP contexts can be opened in the UMTS system. The first context is called a primary PDP context and the next PDP contexts are secondary PDP contexts.
The mobile station knows which application data flows are to be directed to which links of a PDP context in uplink data transfer. In downlink, the gateway GPRS support node should also know packet-specifically which PDP context is used for each data flow received from an external IP network. For this purpose, the destination IP address of the packet is used; TFT templates (Traffic Flow Templates) are also specified in the UMTS. The idea of TFT templates is that the mobile station sends given TCP/UDP/IP header field values to the gateway GPRS support node GGSN for identification of the data flow. A TFT contains one or more so-called packet filters. These packet filters can be used particularly for QoS (Quality of Service) mapping, i.e. mapping received packets into a data flow offering a quality of service according to the QoS information, e.g. the DiffServ field (Differentiated Services), in the UMTS system.
In addition, an IP multimedia subsystem IMS is designed in the UMTS system for providing various IP multimedia services to UMTS mobile stations (UE; User Equipment). The IMS utilizes packet-switched UMTS services, PDP contexts, for data transfer to or from a mobile station. The IMS includes functions that enable negotiation of an end-to-end session on the application plane using the SIP protocol (Session Initiation Protocol), the features of the session being for instance the codecs used, the termination points and the quality of service (QoS). For arranging the agreed end-to-end quality of service also in a UMTS network, the IMS includes a call session control function (CSCF), which includes a PCF function (Policy Control Function) for authorizing quality of service resources (bandwidth, delay, etc.) for an IMS session based on SIP-layer SDP information (Session Description Protocol). For binding the authorization decision, an authorization token is determined in the PDP context, which the PCF creates for each session and which is transmitted from the CSCF to the mobile station. When the PDP context is being activated, the mobile station sends, to the gateway GPRS support node GGSN, an authorization token and a flow identifier that constitute binding information. The flow identifier identifies the IP media flow associated with the SIP session. The GGSN comprises a PEP function (Policy Enforcement Point) that controls the offering of the quality of service resources to the data flow according to the authorization token received from the PCF. The GGSN requests authorization for allocating resources to the session indicated by the binding information from the PCF, which is located at the P-CSCF (Proxy CSCF). The PCF functionality makes a final decision on resource allocation to the session and responds to the GGSN. The PEP function of the GGSN generates, based on this, a logical ‘gate’ for implementing admission control according to the decision of the PCF for a unidirectional data flow. A source IP address, destination IP address, source gate, destination gate and protocol may be used as packet classification parameters.
A problem in the above arrangement is that the GGSN performs similar downlink packet filtering by means of the gate functionality provided both by the TFT templates and by the PEP functionality. If the packets pass through the gate, a PDP context is selected for them by means of the TFT functionality, as illustrated in FIG. 1. Problems arise if these filters do not match, in which case the packets do not end up in the right PDP contexts.